BACKGROUND. Genetic screen using RNAi and CRISPR/Cas9 libraries is a powerful approach to interrogate the cancer genome for functional vulnerabilities. We have developed technologies to conduct rapid and cost-effective shRNA and sgRNA library screens. PURPOSE. In this project we aim to carry out loss-of-function and gain-of-function genetic screens using RNAi and CRISPR/Cas9 libraries to study genetic vulnerabilities in cancer cells and identify mechanisms of drug resistance. We will also collaborate with other investigators (primarily investigators in the NIH Intramural Program) to enable them to conduct screens in cancer cell lines. SIGNIFICANT MATERIALS AND METHODS. Pooled shRNA and CRISP/Cas9 library targeting most annotated human genes. ACCOMPLISHMENT. We have completed several screens in a number of different cancer disease areas. In particular, we have identified genes and genetic pathways that could modulate KRAS mutant cancer cells' sensitivity to MEK inhibitors. We are studying candidate genes from these screens.